Spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry is commonly used to measure the transmittance or reflectance of solutions, transparent or opaque solids, or gases. The device that performs this measurement is known as a “spectrophotometer”.
FIG. 1 depicts a block diagram of a typical prior-art spectrophotometer 108 in use performing a spectral assay of media 104. Spectrophotometer 108 includes Fabry-Perot interferometer 112, detector(s) 116, and processor 120.
Interrogating light 102 emitted from broadband light source 100 is directed towards media 104. The light is dispersed, via reflection, absorption, etc., as it passes through media 104. The dispersion alters the spectral content of the interrogating light. The specifics of the alteration depend on and can be characteristic of media 104. As a consequence, analysis of spectrally altered light 106 can provide information about media 104. This information is “extracted” using interferometer 112, detector(s) 116, and processor 120, as discussed further below.
Spectrally altered light 106 enters Fabry-Perot interferometer 112. Wavelengths of spectrally altered light 106 that resonate within interferometer 112 form filtered exit light 110. In this fashion, interferometer 112 selectively filters spectrally altered light 106.
Filtered light 110 exits interferometer 112 and is directed to detector(s) 116. In some prior-art spectrophotometers, detector(s) 116 are sensitive to certain wavelengths of electromagnetic (EM) radiation and generate electrical signals 118 (i.e., a photocurrent) when such wavelengths are detected. The amplitude of signals 118 is indicative of the light intensity at the particular wavelength. Signal(s) 118 from detector 116 are conditioned (analog-to-digital conversion, etc.) and transmitted to processor 120. In the processor, signal(s) 118 are processed via a Fourier transform or related algorithms to provide assay 124 of the spectral content of filtered exit light 110.
As previously noted, filtered exit light 110 will contain wavelengths corresponding to the resonances of the interferometer cavity. Analysis of those particular wavelengths will rarely provide a complete spectral analysis of spectrally altered light 106. Consequently, as part of the spectrophotometry process, the resonant wavelengths of interferometer 112 are altered. In some spectrophotometers, this alteration is implemented by changing the cavity length of the interferometer, such as via cavity-length controller 114. Each such alteration will change the spectral content of exit light 110. In this fashion, a wavelength sweep is performed, wherein for each change in cavity length (and, hence, spectral content of the exit light 110), the detection operation is repeated. This ultimately provides a complete spectral analysis of media 104 (assuming that the frequency sweep is large enough). The spectral analysis, which provides light intensity as a function of wavelength, can be used as a fingerprint (for identification purposes) and/or as a means to quantify the amount of media that is present. Identification and/or quantification involves a comparison of the spectral analysis to a database that provides compound identification as a function of spectrum or concentration (of a particular media) as a function of spectrum.
An embodiment of conventional Fabry-Perot interferometer 112 is depicted in FIG. 2. Interferometer 112 consists of two spaced-apart mirrors 226 and 228. The mirrors are typically “highly” reflective, such that most of the light impinging on them is reflected. The change in the “thickness” of the lines that are representative of light “beam” is intended to be (qualitatively) indicative of the attenuation of the transmitted intensity resulting from reflections at mirror surfaces.
The portion of light 106 entering interferometer 112A makes multiple (partial) reflections between mirrors 226 and 228. Although depicted as a single coherent beam (like a laser beam), spectrally altered light 106 is in the form of a broad plane wave comprising multiple wave fronts. Constructive interference (resonance) occurs if the transmitted light is in phase, and this corresponds to a high-transmission peak of the interferometer. If the transmitted light is out-of-phase, destructive interference occurs and this corresponds to a transmission minimum.
The resonant wavelengths of a Fabry-Perot interferometer are a function of the angle that light travels through the interferometer, the size of gap between the mirrors (i.e., cavity length) and the refractive index of the medium between mirrors. For fixed values of those parameters, the wavelength of the reflected light determines whether that light is “in phase” or “out-of-phase”.
The resonant wavelengths of a Fabry-Perot interferometer can be altered by changing its cavity length. Cavity length can be changed via cavity-length controller 114 (see FIG. 1), which in interferometer 112 depicted in FIG. 2 comprises electrostatic actuator 230.
Electrostatic actuator 230 includes controlled voltage source 232. Mirrors 226 and 228 are electrically conductive, so that when a voltage is applied across them, an electrostatic force of attraction results. Mirror 226 is suspended (e.g., from a stationary substrate, etc.) via tethers 234 that enable mirror 226 to move. Consequently, when a voltage is applied across mirrors 226 and 228 creating an electrostatic force of attraction, tethered mirror 226 moves toward mirror 228. This movement reduces the size of gap G compared to the quiescent state in which no voltage is applied. Within the range of movement of mirror 226, the size of gap G is a function of voltage. Since, as already indicated, a change in cavity length alters the resonances of the interferometer, the transmission spectrum as a function of wavelength for interferometer 112 can be altered via electrostatic actuator 230.
Most prior-art spectrophotometers are fabricated with minimal integration of elements. This affects cost and also limits the type of applications in which such spectrophotometers can be used.